1. Field of the Invention
The present invention relates to a polarized beam splitter using a diffraction grating and a magneto-optical head using the polarized beam splitter.
2. Description of the Related Art
In general, a polarized beam splitter used for a magneto-optical head is provided by producing dielectric multi-layers of an isotropic medium having a constant refractive index regardless of an incident polarized light beam direction and putting the dielectric multi-layers together onto the slope of a triangle prism such as a polarized light prism 11 in FIG. 1. For a boundary plane of the dielectric multi-layers, a P polarized light beam (a component of linearly polarized light beam vibrating in the plane perpendicular to the boundary plane) can be transmitted 100% and a S polarized light beam (a component of linearly polarized light beam vibrating in the boundary plane) can be reflected 100%. For detecting a polarized light beam rotation in the magneto-optical recording medium by using these polarized light beams, a signal generation is performed by sloping the direction of a linearly polarized light beam of the reflection light beam by 45.degree. for a splitting direction so that a transmission light beam has the same optical amount as reflection light beam and detecting the change of a balance between the transmission light beam and the reflection light beam along the polarized light beam rotation from a difference of a detected amount of light beam.
As another prior method described in a patent application No. 4-19522, there is provided a magneto-optical head using a specific Wollaston prism 21 and configured in a Japanese patent application No. 6-77351. This Wollaston prism 21 is provided by putting an anisotropic medium together in a specified direction of a principal axis and is configured as shown in FIG. 2. In other words, there is a light beam having the same status of polarized light beam as an incident light beam in the center and two divided polarized light beam components crossing orthogonal to both sides. A magneto-optical head can be miniaturized by packaging into a unit because three optical detectors 22, 23 and 24 can be configured in a plane by using this dividing method. It is possible to detect a magneto-optical signal from light beams on both sides and a servo signal such as a focusing signal or tracking signal from a light beam in the center.
Contrary to the polarized beam splitter of a magneto-optical head, a polarized beam splitter using a hologram is described in pages 622 through 628 of National Technical Report, Vol. 41, No. 6, 1995, and shown in FIG. 3. A light beam incident on hologram device 31 and being polarized in a specific direction transmits through without an occurrence of a diffracted light beam, becomes a rotated polarized light beam through a .lambda./4 board 32, reflects at a reflection substance 33, transmits through the .lambda./4 board 32 again, becomes a linearly polarized light beam in the direction crossing orthogonal to what it was at the time of incidence and is incident on the polarizing diffraction hologram device 31. An anisotropic optical crystal such as a lithium niobate is used for a medium to configure the polarizing diffraction hologram device 31 and a configuration that differentiates a phase difference acted by a grating to two linearly polarized light beam crossing each other orthogonally. The polarizing diffraction hologram device 31 can be utilized as a polarized beam splitter 31 that splits a linearly polarized light beam in a polarized light beam direction at the time of incidence as a 0.sup.th order light beam and a linearly polarized light beam in the direction crossing orthogonal to the 0.sup.th order light beam as a .+-.1.sup.st order diffracted light beam.
Although an optical crystal is necessary, since a diffraction hologram is provided, the polarized beam splitter can be produced through an exposure development process using a photo-mask and have an advantage of mass production. Further, it is easy to miniaturize the polarized beam splitter and there is a feature in the beam splitter that its splitting ratio and an angle of splitting can be designed freely.